Wheels
The plans show the back to back distance for the wheels as being 4-5/8" but this is now considered incorrect for running where there are points to be crossed. Modern 5" gauge locomotives now use a back to back distance of 4-11/16" Wheel Profile The plans for SPEEDY clearly show a tapered tread but there are no details for the angle and the original design aparrently had parallel treads. The GL5 Ground Level 5 inch Gauge Main Line Association have published a set of standards that define a suitable profile and back to back dimensions for modern usage. Changes have been made to the back to back dimensions and flange details to allow for safe running over points. Parallel treads can be used with the new back to back wheel dimensions if the radius between the tread and the flange increased. It now seems generally accepted that tapered treads are preferable but there is still some strong opinion against them. Note:- See the section on Axleboxes for the changes to accommodate the longer back to back distance. Note:- Brake blocks will naturally need to match tapered treads. Steel tyres Most model locomotives use wheels made from Cast Iron with the tread profiles turned directly onto the cast material. This is completely satisfactory but some builders like to follow full size practice and fit a steel tyre. The justification for this is mainly to improve the grip but it also increases the life of the tyre. The tyres shown were machined from a piece of EN8 bar. There is an interference of 0.12mm and the tyre has to be heated to a Dark Straw colour (about 150-160C) before the middle is dropped in. The tyre can be roughed to the tread profile before fitting to reduce the amount of material that needs to me removed when machining the completed axle between centres. The front of the tyre has a thin flange which is designed to locate the front of the tyre and give an appearance similar to the full size locomotive. There's a lot of machining and wastage to do this, and it's probably a candidate for 3D printing and a steel casting. BEWARE:- there are three different castings for speedy's wheels, and if you're going to the trouble to make a 3D model, it would be worthwhile measuring the full size locomotive wheels which are significantly different to the castings supplied by Reeves! Counterbalance weights The full size locomotive uses plates that are attached to the spokes with bolts, the space created being filled with the required amount of lead. The coupled wheels are all identical, but the driven wheels had weights that span 5 spokes instead of 4. They are also handed, as in the 3D model image here. The RH axle centre has been marked on the model for clarity. Note the position of the counterbalance weights and the relative positions of the crank pins. The model is of a deviation from the plans with steel tyres fitted for improved traction and cosmetic appearance. There's not enough material in the castings to properly represent the size of the counterbalance weights on the driving wheels on the full size locomotive. The 3D model shows tapped holes in the cast counterbalance portion where additional plates will be added. The plans show a different appearance of the centre of the hub too. On the full size locomotive, the whole crankpin area is spot faced whereas the plans show this as being flush on the outside. Axles The axles are made from ground Mild Steel or Medium Carbon Steel. Silver Steel is not considered suitable, and certainly not if through hardened where cracking may occur. Turning axles between centres provides a way to get them parallel by adjusting the offset of the tailstock. Allow an additional 0.2mm on the length for trueing up when the axle is fully assembled.There should be a fine finish because the axleboxes will run directly on the axle. A clearance of 10-20 microns seem to be acceptable. An interference of around 15microns on the wheels should be aimed for. If it's nearer 20 microns there's a risk of the casting splitting. If it's much less, use permanent Loctite to ensure it stays on. There's no harm in adding a smear of Loctite anyway. It's much easier to control the diameters and surface finishes if the axles are ground. If you want to do this, allow an additional 0.2mm on the diameter and shoulder lengths. The shoulder to shoulder distance should be 119.063mm (4-11/16") When the wheels are pressed home, it's common practice to drill a hole across the joint between the axle and the wheel to stop it from moving. Cast Iron is much softer than steel and there is a tendency for the drill to deflect away from the steel axle. The top picture shows a ball nose slot drill being used to cut a semicircular keyway in the axle. The 'Dog' attached to the end of the axle is there so it can be aligned at 90 degrees so the keyway in the other end is in the same relative position to the wheel. The custom is for the RH crank to lead the left by 90 degrees. The idea is to make sure that the hole drilled in that place follows the groove while drilling the soft cast iron. A pin of a suitable side is pressed into the drilled hole. Loctite can be used to retain this. If you need to turn a pin, one trick is to use say 5mm free cutting mild steel and turning it down to size in one big cut. Leave only just enough protruding from the chuck to keep deflection at a minimum. Turning the pin in one pass allows the large uncut stock to support the smaller diameter. There will still be a small taper on the pin, so make sure the larger diameter (furthers from the chuck) ends up on the outside of the wheel. Quartering Much has been written about this subject and it's something that can be a source of difficulty. Quartering is the process by which the cranks are set at 90 degrees to each other so that the pistons give an equal spacing between power strokes. The precise angle isn't critical, but all of the axles must be the same else the connecting rods will bind when going over the centres. Most quartering methods assume that the crank pins have been pressed in place, but there is an alternative method that uses the holes for them for quartering. The steel fixture shown has two holes to locate the crank pins. Obviously these need to be accurately machine to the same radius as the holes in the wheels. The jig was made from round bar for convenience. The hole for the other crank pin in the bottom of the jig is in the same half of the jig as the one you can see. Two pins were made that were a precision sliding fit in the reamed holes in the wheels and the fixture. The pins have tapped holes in them to assist their extraction once the wheels are pressed home. NB:- You might be tempted to think that the crank throw is not of much importance. The precise radius of the holes is not the issue, it's vitally important that all of the radii are the same on all the wheels though. It's worth taking the time to make a drill jig to guide the reamer to make absolutely certain of this. The reason it's so important isn't immediately clear until you exaggerate the effect. If you draw two cranks, with one at twice the radius to the other then all will become clear. When the connecting rod is in the forward position at dead centre, the distance between the crank pins is different to the same measurement taken at the opposite dead centre. This causes the wheels to bind as they go over the dead centre. The actual difference is twice the difference of the radial error. In other words, if the radius of one crank pin is 0.1mm larger than the one on the other wheel, the connecting rod will be 0.1mm too long at one dead centre and 0.1mm too short at the other. The only fix for this is to measure the error and to make an eccentric crank pin to bring it to the same radius. It's a lot easier to take time to make a jig and get this right compared to fixing it when it's wrong. Drive pins NB:- The leading wheel drive pins are shown with a 1/4" long journal but that's 1/32" shorter than the bush in the connecting rod. There's also no end clearance which can be another 1/64" The drive pins for the connecting rod can be made of unhardened Silver Steel or Mild Steel which is Case Hardened. The second of these has the advantage of being easier to machine but ultimately harder wearing. Through hardening of any steel isn't recommended as it may crack in service. Make sure you allow a little extra material on the case hardened part (say 15-20microns) for the loss of diameter due to the case hardening process. The press fit rule of thumb of 1/1000 of the diameter can be used for the initial calculation. It may be advisable to thoroughly clean the drive pins and wheel holes with solvent and adding Loctite retaining compound to both surfaces before pressing home. Some builders recommend pinning the drive pin on the drive axle but this is probably not necessary if Loctite is used. References 'Manual of Model Steam Locomotive Construction' by Martin Evans 'The ground level 5 inch gauge main line association' web site Category:Smokebox